Thyrotropin (TSH) comes from the anterior pituitary gland and is the major controlling factor for the development and function of the thyroid gland which is the essential supplier of thyroid hormone to the entire body. TSH works on the thyroid gland through the TSH receptor (TSHR) which is a complex molecule expressed on the thyroid cells as well as on a variety of non-thyroid tissues including fat and bone. The TSHR is also a major site of attack by the immune system in some common diseases such as Hashimoto's thyroiditis and Graves' disease and becomes overactive in what we call hot thyroid nodules causing thyroid over-activity. The TSH receptor is also involved in thyroid cancer since in animal models the lack of the receptor dampens the propensity for cancer development. The complexity of the TSH receptor is further enhanced by the fact that it has a variety of structures secondary to the formation of variants following RNA splicing. However, the splicing repertoire of the TSH receptor has not been well characterized since our early description of what we call variant 1.3. Although we know that increased diversity of cell signaling options at the receptor can be enhanced by increased numbers of receptor forms, the functional consequences of TSH receptor splicing, cleavage and multimerization in the thyroid and in non-thyroid tissues remains incomplete. Hence, the aims of the proposed study are: (1) To characterize TSH receptor variants in normal and pathological tissues and their role in modulating receptor function. (2) To examine the influence of stimulating the TSH receptor on the variants and multimers. (3) To identify TSH receptor variants and multimer formation in non-thyroidal tissues. This grant proposal is to advance our understanding of the structure and function of the TSH receptor variants and to see how they influence signal bias and diversity through multimerization. This knowledge will help us understand the role of the TSH receptor in a variety of thyroid disorders including thyroid nodules, thyroid cancer and autoimmune thyroid disease.